The present invention relates to a light emitting device used for a display device, in particular, to a light emitting device which can suppress the growth of a so-called dark spot.
A light emitting element called an organic electroluminescent element is conventionally known. In such an element, an organic electroluminescent (EL) thin film is interposed between electrodes, and by applying a voltage between the electrodes, carriers are injected to the organic EL thin film, thus generating light.
Generally, an organic EL element has a structure in which an anode (electrode) which is a transparent element, an organic EL layer and a cathode (electrode) made of a single mental or a metal alloy, are formed in the order, on a substrate made of glass or the like.
In the process of manufacturing the organic EL element, if there is a particle (small piece of trash, dust, undesired material or the like, these will be called particle hereinafter) present, in particular, on the electrode (anode electrode) immediately before the formation of the organic EL layer, the particle remains in the interface between the anode and the organic EL layer formed thereon.
In case where the organic EL layer is formed while a particle remains as described, the following problems easily occur.
First, the thickness of the organic EL layer is usually 500 nm or less; however the intensity of the electric field applied in order to inject carriers to the organic EL layer becomes extremely high as about several thousand V/m. Consequently, with the presence of a particle which usually exceeds 1 .mu.m in size, short-circuiting easily occurs between the electrodes. Due to the short-circuit between the electrodes, the sections of the cathode and organic EL layer, where the particle was present, were damaged, thus causing a pin hole.
In general, as compared to an anode, a cathode has a tendency of being easily oxidized since it is made of a metal having a high electron injection property. When oxygen or moisture enters the cathode from the pin hole, the cathode is oxidized, and the work function is significantly increased. Further, when the cathode is oxidized to a certain thickness, it becomes difficult to inject electrons to the organic EL layer. As a result, there would be a section created in the organic EL element, where light is not generated even if a predetermined voltage is applied (the section will be called dark spot hereinafter). In addition, the cathode is oxidized in a radial manner with the pin hole serving as the nucleus, and therefore the dark spot grows in a radial manner in the surface direction along with time, from the firstly generated spot as the nucleus.
In order to avoid such a problem, the following method is conventionally employed. That is, the organic EL element is coated with a sealing member such as of resin, or another substrate is provided above the organic EL element, and silicone oil is sandwiched between these substrates, so as to inhibit oxygen and moisture from entering the cathode. However, with the sealing member or silicone oil, it is not possible to fully avoid the entrance of oxygen and moisture to the cathode, and therefore the creation and growth of a dark spot cannot be completely suppressed.
In the case of an organic EL element in which a cathode is divided into pieces for each one of pixels, which is a light emitting region of the minimum unit, once a dark spot is locally created on the cathode of a pixel, eventually, the entire pixel is covered with the dark spot. In the case of an organic EL element having a simple matrix structure in which one cathode is formed continuously over a plurality of pixels, as a common electrode, the oxidization of the cathode occurs not only on the pixel in which a pin hole is created, but also expands to an adjacent pixel or pixels.
Therefore, the number of pixels which do not generate light increases gradually. Thus, the size of the dark spot increases along with time, and the display vision quality of the organic EL element is extremely deteriorated.